How did the Morlocks overcome the Eloi?
In Chapter Seven of The Time Machine, the Time Traveller makes a startling discovery about the Morlocks and the Eloi:
And for the first time, with a sudden shiver, came a clear knowledge of what the meat I had seen might be.
This is the critical realisation that the Morlocks hunt and eat their neighbours, the Eloi. This explains the "Great Fear" which the Eloi demonstrate whenever the Time Traveller tries to ask questions about who might have taken his time machine and who dwells in the darkness below.
Based on his early observations, the Time Traveller believes that this society split into the Haves and Have-Nots at one time in the past:
So, in the end, above ground, you must have the Haves, pursuing pleasure and comfort and beauty, and below ground, the Have-Nots, the workers getting continually adapted to the conditions of their labour.
Eventually, the Morlocks ran out of food and looked to their neighbours above for a fresh source of meat:
The Eloi were mere fatted Cattle, which the ant-like Morlocks preserved and preyed upon.
The reader sees how the Morlocks have adapted to suit their hunting of the Eloi. They have "large, bright eyes," for example, which enable them to better see the Eloi in the darkness. They are expert climbers, described as a "human spider" by the Time Traveller (Chapter Five). Again, this agility is what helps them to hunt efficiently. Likewise, the Eloi have evolved in a manner which makes them easy prey: they are "fragile" and unintelligent, and exhibit a "childlike ease," says the Time Traveller in Chapter Four. Thus, the Morlocks and the Eloi have adapted to become the hunter and the hunted.